Salicylic acid is a BHA (beta hydroxy acid) and it's oil-soluble. Generally, oil-soluble ingredients penetrate through the lipid layers between the skin cells more readily. In other words, oil-soluble ingredients can penetrate the skin at a deeper level than their water-soluble counterparts. Salicylic acid works deeper [and is] able to penetrate into the pores to unclog them. This quality is precisely what makes salicylic acid such a potent ingredient for targeting acne — especially for blackheads and whiteheads.
Salicylic acid is also an exfoliant. It loosens and breaks apart desmosomes (attachments between cells in the outer layer of skin) which encourages exfoliation of skin and unclogging of pores. Salicylic acid works best on blackheads and whiteheads as it can directly dissolve the keratin plugs and regulate the skin cells.